Chemistry: electrical and wave energy – Processes and products
Patent
1994-07-11
1995-12-26
Gorgos, Kathryn
Chemistry: electrical and wave energy
Processes and products
204 59R, 205162, 205915, C25C 122
Patent
active
054784450
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the production of chin film semiconductor materials by electrodeposition.
Thin film semiconductors are of value in the fabrication of photovoltaic cells. The production of semiconductor problems by electrodeposition gives rise to special problems. It is necessary to prepare materials of electronic grade purity, as very small quantities of impurity can have a large effect on the performance of devices made from the semiconductor material. The purity of the materials required is much higher than that for example needed in conventional metal plating.
One class of semiconductor materials which have been deposited by electrodeposition are the IIB/VIB semiconductor materials. Thus the production of thin film IIB/VIB semiconductors, e.g CdTe, by electrodeposition is disclosed by Panicker, Knaster, and Kroger, "Cathodic Deposition of CdTe from Aqueous Electrolytes", J. Electrochem. Soc. 124, No 4, April 1978 pp 556-572. Deposition of CdTe cakes place from an aqueous solution of CdSO.sub.4 to which TeO.sub.2 had been added and electrolysis was carried out using a nickel plate or glass places covered with semiconducting tin oxide:antimony material on which the CdTe was deposited. Two anodes used were used together. One was a graphite rod and the other was a rod of Te.
The production of a CdTe layer by electrodeposition and its use in the fabrication of photovoltaic cells is disclosed in U.S. Pat. No. 4,425,194. Various arrangements of electrochemical cell are disclosed, for example one in which the anode is a tellurium bar and another in which the anode-is an inert carbon or stainless steel anode, or both a cadmium and a platinum anode, described as a neutral anode.
U.S. Pat. No. 4,548,681 discloses the electrodeposition of cadmium mercury telluride from aqueous solutions containing Cd.sup.2+, HTeO.sub.2.sup.+ and Hg.sup.+ ions. The principal anode is a Te anode but a graphite anode is also provided.
The common types of "inert", "neutral" or "non-consumable" anode are carbon anodes or are based on platinum, such as platinum metal or platinized titanium, e.g. titanium metal coated with platinum.
Although layers of II/VI semiconductor which can be used to make photovoltaic cells, such as CdTe, can be deposited when using such anodes we have found that the photovoltaic devices made from such thin films have impaired properties. For example devices suffer from low open circuit voltage (V.sub.oc) and low fill factor (F.F.), and tend to have inconsistent and irreproducible behaviour.
We have found that in small scale production for short periods of time the purity of the materials used to make the electrolytic bath is of importance. However in larger scale production over prolonged periods of time we have found that the anode is a significant source of problems with semiconductor purity.
We have now found a method of depositing thin films of semiconductor electrolytically which enables improved material which can be used to make devices, e.g. photovoltaic cells of improved performance.
According to the present invention the process for the formation of a thin film of a semiconductor by a process which includes electrolytic deposition of material from a bath by passing current between an anode and a cathode is characterized in that the anode is separated from the bath from which the semiconductor is deposited by an ion-exchange membrane so as to give an anolyte compartment and a catholyte compartment.
The semiconductor is preferably a IIB/VIB semiconductor, i.e. a semiconductor containing at least one element from Group IIB and at least one element from Group VIB. In this specification references to IIB and VIB are references to the Periodic Table of the Elements as appearing in "Advanced Inorganic Chemistry" by Cotton & Wilkinson, 4th Edition, in which Group IIB includes Cd, and Group VIB includes Se and Te. The preferred semiconductors are compounds of Cd and Te, which may also contain Hg, as disclosed in U.S. Pat. No. 4,548,681. In addition to compounds of Cd,Te, and Hg it is also possib
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Morris et al., Towards High Efficiency Electrodeposited Cds/CdTe Thin Film Cells, Conference Record, 21st, IEEE, Photovoltaic Specialiots Conf. (May 1990), pp. 575-580.
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Barker Jeremy
Marshall Rodney J.
Sadeghi Mehran
BP Solar Limited
Gorgos Kathryn
Wong Eana
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